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FreeBSD-EN-20:02.nmount                                         Errata Notice
                                                          The FreeBSD Project

Topic:          nmount invalid pointer dereference

Category:       core
Module:         kernel
Announced:      2020-01-28
Credits:        Andrew Turner
Affects:        FreeBSD 11.3 and FreeBSD 12.0.
Corrected:      2019-10-03 07:17:26 UTC (stable/12, 12.1-STABLE)
                2019-10-04 14:10:56 UTC (releng/12.1, 12.1-RELEASE)
                2020-01-28 18:54:15 UTC (releng/12.0, 12.0-RELEASE-p13)
                2019-10-04 17:27:49 UTC (stable/11, 11.3-STABLE)
                2020-01-28 18:54:15 UTC (releng/11.3, 11.3-RELEASE-p6)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

The nmount(2) system call is used to mount file systems.

II.  Problem Description

VFS option processing related to the nmount(2) system call was missing a
length check.  This is generally only available to privileged users unless
the vfs.usermount sysctl is changed from the default to allow non-privileged
users the ability to mount file systems.

III. Impact

A userland process authorized to mount filesystems can possibly trigger a
kernel panic.

IV.  Workaround

No workaround is available.  However, if the vfs.usermount sysctl has been
changed to allow non-privileged users the ability to mount file systems,
switching back to the default value of 0 will prevent non-privileged users
from triggering the issue.

V.   Solution

Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date, and reboot.

Perform one of the following:

1) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for an errata update"

2) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-20:02/nmount.patch
# fetch https://security.FreeBSD.org/patches/EN-20:02/nmount.patch.asc
# gpg --verify nmount.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r353032
releng/12.1/                                                      r353104
releng/12.0/                                                      r357216
stable/11/                                                        r353109
releng/11.3/                                                      r357216
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-20:02.nmount.asc>
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